Sound transducer

ABSTRACT

A sound transducer has an inertial mass and one or more acoustic transducer plates fastened along an edge to the mass in cantilever fashion. The plates respond to acoustic energy at their main surfaces.

United States Patent [56] References Cited UNITED STATES PATENTS l 1/1953 Baerwald................v...

[72] Inventor Robert H. Dreisbach Fort Wayne, Ind. 787,301

2,659,829 2,978,597 4/1961 Harris.

[Zl Appl. No. [22] Filed Dec. 18, 1968 [45] Patented Sept. 7, 1971 Primary Examiner-Richard A. Farley Assistant ExaminerBrian L. Rib Attorney-James J. Williams ando m D m 70 H mm w m an. n mm m m C. XMHH o o u m m mm 8M7 M m mm; TFCS e e n .m s A H 7 [54] SOUND TRANSDUCER 18 Claims, 12 Drawing Figs. [52] US.

ABSTRACT: A sound transducer has an inertial mass and one or more acoustic transducer plates fastened along an edge to the mass in cantilever fashion. The plates respond to acoustic energy at their main surfaces.

[5 l] Int. [50] Field 0! LENGTH PATENTEDSEP nan sum 1 or 4 INVENTOR 1171 a.

WIDTH PATENTEUSEP 11ml V 3.603921 SHEET 30F 4 LENGTH I DIRECTION PATENTEDSEP 7l97| SHEET U [1F 4 INVENT OR flwma (r141,

SOUND TRANSDUCER This application is a continuation application of my copending application filed Nov. 21, 1966, entitled Improved Sound Transducer," Ser. No. 596,007 now abandoned.

My invention relates to an improved electroacoustic transducer and particularly to an improved electroacoustic transducer for use in liquids.

Electroacoustic transducers are used in liquids for converting applied electrical energy into acoustical energy for transmission through the liquid, and for converting acoustical energy in the liquid into electrical energy for the use in an electrical circuit. When operated in the latter mode, such transducers are commonly referred to as hydrophones and are extensively used in water for detecting acoustical energy which may be relatively weak and which may have frequencies covering a relatively wide band.

In certain applications, it is desirable that the transducer or hydrophone possess directivity characteristics so that the relative direction of the source of the acoustical energy may be determined.

In the past, single transducers have been used which possess such directivity; however, their operation at the lower frequencies is not generally satisfactory especially with regard to their directional characteristics. Parabolic reflectors and the like have also been used with some success; however, their sensitivity or gain and directional properties are a function of reflector size which ideally should be large compared to the acoustical wavelength. This therefore places a minimum size restriction on such type transducers especially where the application requires good low-frequency operation. In addition, an ideal hydrodynamic configuration is not usually inherent with such directional transducers and as a result they exhibit a greater undesired resistance to the current flow of the surrounding liquid and this is sometimes accompanied by an increase in the ambient noise output level of the transducer.

Other methods of obtaining directivity have been used whereby two or more omnidirectional transducers are placed apart from one another in a spaced relationship and electrically connected so as to produce an array with the desired directional characteristics. Such arrays normally measure the acoustical pressure differential between the spaced positions and therefore require that each of the separate transducers be very closely matched in sensitivity at all operating environments, such as temperature and static'pressure. This matching is extremely difficult to accomplish, measure, and control especially when large production quantities are involved. Because such transducers are normally of the pressure operated type, their sensitivity under high static pressures, unless otherwise compensated for such pressures, is low. In addition', since such arrays operate upon the difference in the acoustical pressure between the spaced transducers, their differential output level is relatively low especially at the lower frequencies where such spacing is generally much less than one-half the acoustical wavelength. This, of course, requires that the electrical output of the transducers be followed by high gain, low noise amplifiers.

Prior art and especially spaced directional arrays are difficult to package and require complicated and sometimes unreliable mechanisms for their deployment especially when deployed from small carrying canisters such as might be required for air launched devices.

Accordingly, an object of my invention is to provide an improved sound transducer particularly for use in water.

Another object of my invention is to provide an improved sound transducer or hydrophone for producing electrical signals in response to sound energy that covers a relatively wide band of frequencies, the electrical signals having charac-.

teristics indicative of the characteristics of the acoustical ener- Another object of my invention is to provide an improved sound transducer that is relatively rugged andstrong, and that is capable of functioning under various marineand environmental conditions.

Another object of my invention is to provide a transducer device having directional characteristics which is easily packaged and deployed.

Another object of my invention is to provide a transducer device having a good hydrodynamic configuration.

Another object of my invention is to provide a transducer device which is not only easily fabricated but which has directional characteristics that are not dependent upon the operational frequency or upon the matching of the sensitivity of separate transducers as in spaced directional arrays.

Another object of my invention is to provide a transducer whose output is a function of the acoustic particle velocity of the surrounding transmission medium.

Another object of my invention is to provide a transducer which exhibits constant sensitivity regardless of the static pressure exhibited by the surrounding transmission medium.

Another object of my invention is to provide a transducer whose sensitivity is independent of operating frequency.

Briefly, these and other objects are achieved in accordance with my invention by a transducer having an inertial mass or base suitable for anchoring one or more transducer elements thereto and of a material of substantially high inertia with relation to that of the transducer element or elements loaded by the transmission medium; such mass or base may comprise a material or combination of materials which in addition to providing a suitable mounting mass also provides a substantially high absorption coefficient to control or dampen acoustical or mechanical vibrations. One or more particle velocity operated transducer elements, preferably of the piezoelectric type, are mounted on or clamped to the said inertial mass. Each transducer element comprises relatively thin plates in a bilaminate construction with electrodes on the main opposite sides. The transducer element is fastened along one edge to the inertial mass in cantilever fashion so that approaching or impinging acoustical energy whose wave front is parallel to the elements main sides, causes a maximum vibration of the transducer plate generally perpendicular to its main sides. This causes the transducer element to produce an electrical signal at its electrodes that is indicative of the direction, magnitude, and frequency of the acoustical energy. Although the single element described, is capable of producing a planar directivity pattern of the familiar figure-8, two such elements when mounted in the same plane, each on directly 0pposite sides of the inertial mass may be interconnected to produce the same type pattern with improved symmetry and increased sensitivity. In addition, more than one such element or pairs of elements may be mounted about the inertial mass in any desired or spaced relationship thereby resulting in one or more similar positioned planar figure-8" directivity patterns or a combination of such patterns to provide the desired directional information. The planar figure-8" pattern or patterns may, if desired, be combined in suitable external circuits with the output of an omnidirectional transducer to provide cardiod-type directional pattern or patterns. The transducer elements may be shaped to provide the desired frequency response or band of frequency response. The inertial mass and transducer elements are preferably incapsulated in a suitable material that protects the transducer elements and in addition transmits acoustic energy with relatively low loss, affords suitable damping, and provides a good impedance matching between the transducer elements and the liquid medium. The material is shaped to provide a good hydrodynamic configuration with little or no distortion of the directive pattern. The electrodes of the transducer elements may be connected in any desired way to an electrical circuit for utilizing the electrical signals.

The subject matter which I regard as my invention is par-' ticularly pointed out and distinctly claimed in the claims. The structure and operation of my invention, together with further objects and advantages, may be better understood from the following description given in connection with the accompanying drawings in which:

FIG. 1 shows a plan view of an improved acoustic-trans-' ducer in accordance with my invention;

FlG. 2 shows an elevation-view of the transducer of FIG. 1-;

FIG. 3 shows an enlarged cross-sectional view taken along the lines 33 of FIG. 2;

FIGS. 4a through 4d show simplified equivalent electrical circuits of the transducer element used in the arrangement of FIGS. 1, 2 and 3;

FIG. 5 shows a plan view of another embodiment of the transducer in accordance with my invention;

FIGS. 6 and 7 show further embodiments of transducer elements in accordance with my invention;

FIG. 8 shows a typical planar directivity pattern which can be obtained by the transducer of FIG. 1; and

FIG. 9 shows a perspective view of the transducer of FIG. 1 with a portion of the incapsulating material removed showing the addition of an integrally incapsulated nondirectional transducer.

FIGS. 1 and 2 show plan and elevation views respectively of sound transducer 10 in accordance with my invention. The transducer 10 comprises an elongated inertial mass 12 formed of a suitable metal, such as copper, that has a square cross section, as shown in FIG. 1; however, the inertial mass may have other or varying cross sections relative to its longitudinal axis. The inertial mass 12 extends longitudinally for a suitable distance as shown in FIG. 2. Four rectangular transducer elements 14, 15, 16, 17 are mounted at the four corners of the inertial mass 12. These transducer elements 14, 15, l6, 17 comprise sheets of piezoelectric material constructed such as a Bimorph (trademark) bender element. Such elements are known in the art, and are described beginning on page 170 of Acoustics by Leo L. Beranek, McGraw-I-Iill Book Company, Inc. 1954. The main planes of these transducer elements 14, 15, 16, 17 extend through a respective corner and the longitudinal axis of the inertial mass 12 so that two transducer elements l4, l6 lie in one plane, and so that the other two transducer elements 15, 17 lie in a second plane that is perpendicular to the first plane. The transducer elements 14, l5, 16, 17 are most sensitive to sound energy in directions perpendicular to their main planes and least sensitive to sound or acoustical energy in directions parallel to their main planes. Therefore, it is preferred that the first plane be perpendicular to the second plane so that the elements 14, 15, 16, 17 can detect sound or acoustical energy for 360 around the transducer 10 as shown in FIG. 8. This embodiment thus produces two of the familiar figure-8" directivity patterns having mutually perpendicular axis which can be used to sense direction with l80 ambiguity or which when properly combined with a nondirectional pattern of an omnidirectional-type transducer will allow the direction of the sound or acoustic source to be sensed or determined without actual rotation of the transducer 10.

FIG. 3 shows an enlarged cross-sectional view, taken along the lines 3-3 of FIG. 2, of transducer element 17, its construction, and its mounting. The other elements 141, 15, 16 are similar in construction and mounting. As shown in FIG. 3, each corner of the inertial mass 12 has a notch or groove in which a respective transducer element is fastened or mounted. In a preferred embodiment, each of the transducer elements, as exemplified by the transducer element 17, comprises two transducer sheets 19, 20 or piezoelectric material such as lead zirconate, electrically connected to opposite sides of an inner electrode 21 of suitable electrically conductive material such as a thin sheet of silver. The transducer element 17 has electrodes 22, 23 which may comprise a plating of silver on the outer faces of the transducer sheets 19, 20. Suitable leads 25, 26 are connected to the electrodes 22, 23 in order that the transducer element 17 may be connected to an external circuit. The transducer element 17 is fastened in the groove in the inertial mass 12 by any suitable bonding or adhesive material 29, which for example, may be an epoxy.

FIG. 4 shows simplified equivalent electrical circuits representing the transducer element 17 of FIG. 3. In FIG. 4a, the voltage generator 37 represents the voltage developed by the piezoelectric material 19 of FIG. 3 and the capacitor 36 represents that capacitance of the outer electrode 22 and inner electrode 21 with the associated piezoelectric material 19 of FIG. 3 acting as a dielectric. The voltage source and capacitor 38 of FIG. 4a likewise represents the piezoelectric material 20 and associated electrodes 21 and 23 of FIG. 3. The circuit of FIG. 4a may be further simplified by combining the voltage sources and capacitors, thus resulting in the circuit of FIG. 4b consisting of voltage source 39 and capacitance 40.

An examination of FIG. 3 and the equivalent circuits of FIG. 4 indicates that the transducer element 17 can have an appreciable capacitance. This capacitance results in part from the relatively large area of the electrodes 21, 22, 23 and the thin sheets of piezoelectric material 19, 20 which separate the aforementioned electrodes. In accordance with my invention, I have found that this capacitance may be reduced, the reason for which will be later apparent, without lowering the magnitude of voltage delivered by the transducer element 17, by breaking or separating the outer electrodes 22, 23 along thin widthwise lines or grooves 30, 31. These separating grooves 30, 31 extend along the full width of the outer electrodes 22, 23 and electrically separate the electrodes 22, 23 into two parts. The lines 14a, 16a in FIG. 2 shows the separating lines or grooves for the front of the two elements 14, 16. In one preferred embodiment of an actually constructed transducer element 17, these grooves 30, 31 preferably occur at a distance approximately equal to 0.42 of the transducer element length measured from the inertial mass to the outer edge of the transducer element as illustrated in FIG. 3 by the length dimension. When the transducer element is mounted or clamped at one end in cantilever fashion as shown in FIG. 3, the location of maximum bending stress occurs near the clamped end with zero bending stress occuring at the free or unclamped end. Since the generated voltage is a function of the degree of bending stress of the element, it follows that only those portions of the element which are subjected to such stress are effective in producing usable voltage and the other portions of the element which are relatively free from such stresses contribute little or nothing to the total produced voltage. This therefore allows the placement of the electrode plates only at the effective stress area thereby resulting in a considerable decrease in the transducers internal capacitance with an actual increase in its output voltage. This is exemplified by FIGS. 40 and 4d. In FIG. 4c, the transducer element 17 is considered as being made up of many paralleled individual voltage sources 41 and associated capacitors 42, 43 along the elements width dimension. The capacitors 43 represent that capacitance which exists at those minimum stress areas of the element where little or no voltage is produced. The voltage sources 41 and associated capacitors 42 represent those which exist at the effective areas of the element where stress occurs and substantial voltage is produced. FIG. 40 is further simplified in FIG. 4d by combining the voltage sources and capacitors. From FIG. 4d, it can be seen that the voltage which is delivered to the external load is that voltage existing across the capacitance 43 of the capacitor voltage divider consisting of capacitors 4 2 and 43. It is therefore evident that that portion of the element's capacitance represented by capacitor 43 may be reduced or removed without lowering the voltage produced by the active portion of the element 17 and delivered to the external load and may in fact because of the aforementioned voltage divider action, increase that voltage delivered to the external load. This therefore allows the placement of the electrode plates only at the effective stress areas thereby resulting in a considerable decrease in the undesired capacitance loading of the usable output voltage of the element 17. In FIG. 3, the maximum element stress occurs lengthwise between the separating lines 30, 31 and similar electrode separating lines and grooves 32, 33 at the inertial mass end of the element 17. Hence the leads 25, 26 are connected to the effective electrode areas 22, 23 with the other portions of the electrode 22a, 22b, 23a, 23b removed from the electrical circuit. These portions of the electrodes may if desired, be completely removed from the piezoelectric material; however, in the specific embodiment described, they are retained as a matter of manufacturing convenience. Under certain conditions where the bonding or adhesive material 29 is electrically conductive, the separating lines or grooves 32, 33 may also be used to prevent short circuiting of the outer electrodes 22,23.

In some instances, it may be desirable to control or vary the sensitivity of one or more of the elements without affecting a change in the area of the element exposed to the transmission medium and in such instance it has been found that for example, the elements sensitivity may be reduced by removing a portion of the electrode plates from the effective stress or voltaging producing areas. Such removal may be accomplished by grooves similar to those previously described or by the actual physical removal or lack of said portions of the electrode plates.

Sound or acoustical waves or energy which strike the main plane of the bilaminate transducer element 17 at an angle somewhat greater than 0 cause the transducer element 17 to vibrate back and forth in cantilever fashion about its point of fastening. These vibrations cause one side of the sheets of the transducer material to be in compression and the other side of the sheets of transducer material to be in tension so that a volt age is produced by each of the corresponding sheets. These voltages are added and appear at the electrodes 22, 23 and have a magnitude proportional to the RMS particle velocity of the acoustical or sound energy. This voltage output is also a cosine function of the angle of incidence of the sound or acoustic wave front. This voltage which is produced at the electrodes 22, 23 may be coupled by the leads 25, 26 to any suitable electrical circuit. The voltages produced by the four transducer elements 14, 15, 16, 17 may be combined in any suitable way. In one preferred embodiment, the voltages from the transducer elements 14, 16 were added in series with each other, and the voltages of the transducer elements l5, 17 were added in series with each other. These two voltages may be used in any suitable electrical circuit to indicate direction or they can be combined as previously described with appropriate voltages from other transducers to eliminate direction ambiguity or modify the directivity pattern.

The bilaminate transducer elements described and used in accordance with my invention provide, in addition to those previously described, other distinct and desirable features which I believe are unique in transducer applications especially of the underwater or hydrophone type. Because the transducer element as shown in FIG. 3 can be made to very small thickness dimensions, such an element exhibits a compliance or mechanical impedance which affords a good acoustic match or coupling between the transducer element, itself, and the surrounding water. The physical dimensions and mode of operation of the transducer element will affect the natural resonant frequency of the element and for a given element size the natural resonant frequency will be lower than when operated either in the length or thickness mode. In accordance with my invention, operation in the bender mode will allow a selection of the resonant frequency which can of course be one optimized for low-frequency frequency operation of the transducer, if such is desired, or can be one selected at any frequency either within or outside of the desired frequency band of operation.

The transducer element 17 clamped at one end as shown in FIG. 3 not only provides a lower fundamental resonant frequency than the same size element clamped at both ends but also exhibits a lower number of overtones within a given frequency range than when clamped at both ends. The effective mechanical Q of the element at the resonant or overtone frequencies may be damped or otherwise controlled by the incapsulating material 35 of FIG. 1 or in lieu of, by a suitable coating applied to the outside of each individual transducer l4, l5, 16, 17. This thus allows the construction of a transducer which provides a substantially wide frequency response.

The transducer shown and described in connection with the various figures is intended to be used in liquids, particularly large bodies of water. Therefore, it is desirable to protect the various elements of the transducer. This may be done in accordance with my invention by a suitable enclosing or incapsulating material placed around the transducer 10. It is preferable that the material by symmetrically shaped and positioned around the transducer 10 so that symmetrical sensing characteristics in at least one plane are provided. Hence, the material 35 preferably has the cylindrical form as shown in FIGS. I, 2, 9. This protective material is preferably a flexible resinous material which has the desired attributes of transmitting sound energy with relatively low loss and which also provides a relatively good impedance match between the transducer elements I4, I5, I6, l7 and the surrounding water. In some application, this impedance match or relation may be better than or preferred over the impedance relation provided where the elements 14, 15, 16, I7, with suitable electrical insulation, are placed directly in the liquid. However, this is a matter of choice and in some applications may not be desirable. FIG. 9 shows one particular embodiment of my invention wherein, an omnidirectional hydrophone is integrally incapsulated with the directional hydrophone. In FIG. 9, part of the incapsulating material has been removed to better illustrate the typical placement of the two hydrophones.

FIG. 5 shows a plan view of another sound transducer 50 in accordance with my invention. The embodiment shown in FIG. 5 is similar to the embodiment of FIGS. 1 through 3. The transducer 50 comprises an inertial mass 51 of any suitable material which may have a square cross section as shown in the plan view of FIG. 5, and extending for any suitable length. Transducer elements 52, 53, 54, 55 are fastened to the inertial mass 51 in the same fashion as described in connection with FIGS. I, 2, and 3. However, these transducer elements 52, 53, 54, 55 are respectively positioned midway along the four sides of the inertial mass Sll rather than at the four corners. Thus, FIG. 5 simply shows another physical configuration or position for the transducer elements 52, 53, 54, 55. It should be noted that the inertial mass 12 and the inertial mass 51 may have other configurations or cross sections, such as a circular cross section, or a hexagonal cross section, or some other preferred or desired cross section or may have a nonuniform or varying cross section along its longitudinal dimension.

Persons skilled in the art will appreciate that the rectangular configuration of the transducer elements shown and described in connection with FIGS. 1 through 5 have a fundamental resonant frequency which depends, among other things, upon the physical length (i.e., distance from the inertial mass to the outer edge), and thickness (i.e., the distance between the electrodes). The physical width (i.e., the distance along the inertial mass) of the transducer elements primarily determines the area exposed to the sound energy, and hence the amplitude of vibrations and the voltage magnitude. The width is indicated in FIG. 2 and the length and thickness are indicated in FIG. 3. In FIGS. 1, 2, and 3, the transducer elements 14, l5, 16, 17 are shown to be the same size, and all therefore have substantially the same fundamental resonant frequency and peak responses. The effect of this resonant frequency and peak responses may be reduced or controlled by damping. The material surrounding or incapsulating the transducer 10 is im portant in this respect. However, even with such damping, the transducer elements have a relative limited frequency response. Accordingly, FIG. 6 shows a transducer element 60 which may comprise the same material and construction as the transducer elements shown and described in connection with FIGS. I through 5. However, the transducer element 60 has a length that varies from a minimum at the bottom to amaximum at the top. When mounted, the transducer element 60 of FIG. 6 has a varying length dimension from the inertial mass, so that various portions have different resonant frequencies of response. In this connection, the shorter length dimensions have higher frequencies of resonance, and the longer length dimensions have lower frequencies of resonance. As described in connection with FIGS. ll, 2, and3, the transducer element may have its electrodes broken or separated along a groove or line 611 in order to decrease the capacity without affeeting the output voltage, and along a groove or line (not shown) at or near the inertial mass.

FIG. 7 shows another arrangement in accordance with my invention for providing a relatively wide band of response. FIG. 7 comprises a transducer element 70 having five subelements 71, 72, 73, 74, 75 which extend from a common portion 76 which would be fastened to an inertial mass. These subelements 71, 72, 73, 74, 75 may also have their electrodes separated as indicated by the grooves or lines 77 thereon. Further separation of the subelements 71, 72, 73, 74, 75 and the common portion along lines at or near the inertial mass (not shown) may be made, as previously described. In both FIGS. 6 and 7, the transducer elements 60, 70 may be mounted in cantilever fashion as shown in connection with FIGS. 1, 2, 3 and 5. in some applications where either a varied frequency response or a relatively wide and substantially flat response is desired, the transducer element 70 of FIG. 7 is preferable over the transducer element 60 of FIG. 6. The frequency band of the individual subelements may be staggered with relation to one another to produce an overall wide band of response. The spacing between the subelements provides decoupling between the said elements and in addition allows each subelement to be damped or controlled to a different and desired degree by, for example, the incapsulation material 35 of FIG. 1. The said spacing may therefore be of the same or different dimensions between the different subelements as may be required to satisfactorily control or flatten the overall response band. The area of the individual subelements 71, 72, 73, 74, 75 and/or the common portion 76 which is exposed to the transmission medium may be varied as desired to further control or flatten the overall response band. While there are many possibilities of the construction and configuration for the subelements 71, 72, 73, 74 and 75, one preferred embodiment uses subelements having respective effective areas which are the same. In this embodiment, the subelements are increased in width by a fixed ratio and their length dimensions are decreased by the same ratio in order that their areas are the same. As previously described this however is not essential, and the subelements may be varied in any desired way.

Sound transducers constructed in accordance with my invention have been found to be very sensitive to relatively weak signals, particularly in the ocean and provide good directional characteristics down to for example, 10 cycles per second. And, the sound transducers constructed in accordance with FIGS. 6 and 7 have relatively wide bands of response. Such characteristics make these transducers extremely useful. And, these transducers make it possible to utilize a number of electrical circuits with them, particularly circuits for determining the direction of the sound energy. While I have described my invention with reference to particular embodiments, persons skilled in the art will appreciate that modifications may be made. For example, only one transducer element may be constructed and mounted in accordance with my invention where this single transducer element provides the desired response and directional characteristic. The elements may be edge mounted to the inertial mass as well as being clamped by or inserted in said mass. The inertial mass may take many forms, and the material surrounding the inertial mass and transducer elements may have different shapes and be of different materials. In the case of the transducer elements which vary in size, such as shown in FIGS. 6 and 7, the variation may follow a number of relations, such as the straight line shown in FIG. 6 or an exponential relation. Likewise, the subelements shown in FIG. 7 may vary in a predetermined relation and any number of subelements may be used. Finally, the material and precise arrangement of the transducer elements may be varied in accordance with circuit requirements. If a high impedance circuit is to be connected to the transducer, it is permissible that the transducer elements be connected to have a relatively high internal impedance. In such a case, two plates of transducer material and series connections are preferred. If a low impedance circuit is to be connected to the transducer, it is 0 and may also provide electrical shielding of the element.

Therefore, while my invention has been described with reference to particular embodiments, it is to be understood that modifications may be made without departing from the spirit ofmy invention or from the scope of the claims.

l claim:

ll. A hydrophone for sensing sound energy in a liquid comprising:

a. an inertial mass;

b. a first relatively thin plate, said first plate having main surfaces of a width and of lengths that vary along said width and having sound transducer material on at least one ofits main surfaces;

c. means fastening said first plate along the width thereof to said inertial mass so that said first plate extends from said inertial mass as a cantilever having variable dimensions; and

d. an electrode coupled to said sound transducer material of said first plate.

2. The hydrophone of claim ll wherein said sound transducer material is piezoelectric.

3. The hydrophone of claim 1 further comprising means for encapsulating said hydrophone.

4. The hydrophone of claim 1 further comprising:

a. a second relatively thin plate substantially similar to said first plate, said second plate having sound transducer material on at least one of its main surfaces;

b. means fastening said second plate along the width thereof to said inertial mass in a different direction relative to said first plate; and

c. an electrode coupled to said sound transducer material of said second plate.

5. The hydrophone of claim 1 wherein said first plate has at least one notch extending from its outer edge towards said inertial mass to further separate said plate into a plurality of plates having different cantilever lengths and resonant frequencies of response.

6. A hydrophone for sensing sound energy in a liquid comprising:

a. an inertial mass;

b. a first platelike element comprising a common portion having surfaces of a length and width, and comprising a plurality of subelements having surfaces of respective lengths and widths, said subelements being spaced along the width of said common portion and extending therefrom in the same direction, said first element having sound transducer material on at least one of its surfaces;

c. means fastening said common portion of said first element to said inertial mass so that said subelements extend from said inertial mass as cantilevers; and

d. an electrode coupled to said sound transducer material of said first element.

7. The hydrophone of claim 6 further comprising means for encapsulating said hydrophone.

8. The hydrophone of claim 6 further comprising:

a. a second platelike element substantially similar to said first element, said second element having sound transducer material on at least one ofits surfaces;

b. means fastening the common portion of said second element to said inertial mass so that said second element extends from said inertial mass in a different direction relative to said first element; and

c. an electrode coupled to said sound transducer material of said second element.

9. A hydrophone comprising:

a. an elongated inertial mass of material;

b. first and second plates of electrical-sound transducer material, said plates being substantially similar and having electrodes, both of said plates having substantially the same width and having lengths which vary along said widths to form transducer elements which have a varying frequency response along said width;

c. and means fastening each of said plates in the vicinity of one edge thereof to opposite sides of said inertial mass so that said plates lie in a common plane and extend symmetrically in opposite directions away from said inertial mass, said plates forming respective cantilevers that respond to sound energy traveling at an angle relative to said common plane and producing electrical signals at said electrodes in response to said sound energy.

10. The hydrophone of claim 9 wherein each of said plates comprise piezoelectrical material.

11. The hydrophone of claim 9 wherein each of said plates has at least one notch extending from its outer edge towards said inertial mass to further separate each of said plates into a plurality of plates having different cantilever lengths and frequencies of response.

12. The hydrophone of claim 10 wherein each of said electrodes is broken along a line that is spaced from said inertial mass at least one-fourth the cantilever length of said plates at each increment along said predetermined width.

13. A hydrophone responsive to sound energy in a medium for producing a voltage in response thereto comprising:

a. an inertial mass;

b. a relatively thin plate having main surfaces bounded by edges, having piezoelectric material on at least one of said main surfaces, and having an electrode connected thereto;

c. means fastening said plate to said inertial mass in the vicinity of one of said edges, said plate extending freely from said inertial mass so that said plate can be vibrated relative to said inertial mass by impinging acoustic energy, thereby causing a voltage to be produced at said electrode; and

d. means for encapsulating said hydrophone.

14. A hydrophone for sensing sound energy in a liquid comprising:

a. an inertial mass;

b. a first platelike element having main surfaces of a length and width and sound-electrical transducer material thereon;

c. a second platelike element having main surfaces of a length and width and sound-electrical transducer material thereon;

. means fastening said first element to said inertial mass in cantilever fashion;

e. means fastening said second element to said inertial mass in cantilever fashion, said first and second elements having different points of attachment to said inertial mass, and extending outward in the same direction with respect to said inertial mass; and

f. electrodes coupled to said sound-electrical transducer material of each of said first and second elements.

15. A hydrophone responsive to sound energy for producing a voltage, said hydrophone adapted to be surrounded by a sound transmission medium comprising:

a. an inertial mass;

b. a relatively thin plate having main surfaces bounded by edges, having sound-electrical transducer material on at least one of said main surfaces, and having electrodes connected thereto;

0. means fastening said plate to said inertial mass, said plate extending from said inertial mass so that said plate can be vibrated relative to said inertial mass by acoustic energy impinging on said hydrophone thereby causing a voltage to be produced at said electrodes in response to any of a plurality of spaced energy sources in said medium; and

d. means for encapsulating said hydrophone. 16. A device responsive to sound energy for producing a voltage in response thereto and responsive to a voltage for producing sound energy in response thereto comprising:

a. an inertial mass;

b. a relatively thin transducer element of piezoelectric material having main surfaces bounded by edges and having electrodes connected to main surfaces, said transducer element producing a voltage at said electrodes in response to forces applied to said main surfaces thereof and producing forces in response to a voltage applied to said electrodes;

c. means fastening said transducer element to said inertial mass in the vicinity of one of said edges so that said transducer element extends free of other connections from said inertial mass in cantilever fashion; and

d. means encapsulating said transducer element.

17. A device responsive to sound energy for producing a voltage in response thereto and responsive to a voltage for producing sound energy in response thereto comprising:

an inertial mass;

a relatively thin transducer element of piezoelectric material having main surfaces bounded by edges and having electrodes connected to said main surfaces, said transducer element producing a voltage at said electrodes in response to forces applied to said main surfaces thereof and producing forces in response to a voltage applied to said electrodes;

means for limiting the electrically effective area of each of said electrode to a value substantially less than the corresponding area of each said main surface; and

means fastening said transducer element to said inertial mass in the vicinity of one of said edges so that said transducer element extends free of other connections from said inertial mass in cantilever fashion; said limiting means comprising nonconductive surface portions abutting said electrodes whereby the voltage to interelectrode capacity ratio characteristic is substantially increased as compared to the same transducer element having coextensive main surfaces and electrodes.

18. The device of claim 17 wherein said limiting means comprises nonconductive surface portions abutting said electrodes and adapted to improve the voltage to interelectrode capacity ratio characteristic as compared to the same transducer element having coextensive main surfaces and electrodes. 

1. A hydrophone for sensing sound energy in a liquid comprising: a. an inertial mass; b. a first relatively thin plate, said first plate having main surfaces of a width and of lengths that vary along said width and having sound transducer material on at least one of its main surfaces; c. means fastening said first plate along the width thereof to said inertial mass so that said first plate extends from said inertial mass as a cantilever having variable dimensions; and d. an electrode coupled to said sound transducer material of said first plate.
 2. The hydrophone of claim 1 wherein said sound transducer material is piezoelectric.
 3. The hydrophone of claim 1 further comprising means for encapsulating said hydrophone.
 4. The hydrophone of claim 1 further comprising: a. a second relatively thin plate substantially similar to said first plate, said second plate having sound transducer material on at least one of its main surfaces; b. means fastening said second plate along the width thereof to said inertial mass in a different direction relative to said first plate; and c. an electrode coupled to said sound transducer material of said second plate.
 5. The hydrophone of claim 1 wherein said first plate has at least one notch extending from its outer edge towards said inertial mass to further separate said plate into a plurality of plates having different cantilever lengths and resonant frequencies of response.
 6. A hydrophone for sensing sound energy in a liquid comprising: a. an inertial mass; b. a first platelike element comprising a common portion having surfaces of a length and width, and comprising a plurality of subelements having surfaces of respective lengths and widths, said subelements being spaced along the width of said common portion and extending therefrom in the same direction, said first element having sound transducer material on at least one of its surfaces; c. means fastening said common portion of said first element to said inertial mass so that said subelements extend from said inertial mass as cantilevers; and d. an electrode coupled to said sound transducer material of said first element.
 7. The hydrophone of claim 6 further comprising means for encapsulating said hydrophone.
 8. The hydrophone of claim 6 further comprising: a. a second platelike element substantially similar to said first element, said second element having sound transducer material on at least one of its surfaces; b. means fastening the common portion of said second element to said inertial mass so that said second element extends from said inertial mass in a different direction relative to said first element; and c. an electrode coupled to said sound transducer material of said second element.
 9. A hydrophone comprising: a. an elongated inertial mass of material; b. first and second plates of electrical-sound transducer material, said plates being substantially similar and having electrodes, both of said plates having substantially the same width and having lengths which vary along said widths to form transducer elements which have a varying frequency response along said width; c. and means fastening each of said plates in the vicinity of one edge thereof to opposite sides of said inertial mass so that said plates lie in a common plane and extend symmetrically in opposite directions away from said inertial mass, said plates forming respective cantilevers that respond to sound energy traveling at an angle relative to said common plane and producing electrical signals at said electrodes in response to said sound energy.
 10. The hydrophone of claim 9 wherein each of said plates comprise piezoelectrical material.
 11. The hydrophone of claim 9 wherein each of said plates has at least one notch extending from its outer edge towards said inertial mass to further separate each of said plates into a plurality of plates having different cantilever lengths and frEquencies of response.
 12. The hydrophone of claim 10 wherein each of said electrodes is broken along a line that is spaced from said inertial mass at least one-fourth the cantilever length of said plates at each increment along said predetermined width.
 13. A hydrophone responsive to sound energy in a medium for producing a voltage in response thereto comprising: a. an inertial mass; b. a relatively thin plate having main surfaces bounded by edges, having piezoelectric material on at least one of said main surfaces, and having an electrode connected thereto; c. means fastening said plate to said inertial mass in the vicinity of one of said edges, said plate extending freely from said inertial mass so that said plate can be vibrated relative to said inertial mass by impinging acoustic energy, thereby causing a voltage to be produced at said electrode; and d. means for encapsulating said hydrophone.
 14. A hydrophone for sensing sound energy in a liquid comprising: a. an inertial mass; b. a first platelike element having main surfaces of a length and width and sound-electrical transducer material thereon; c. a second platelike element having main surfaces of a length and width and sound-electrical transducer material thereon; d. means fastening said first element to said inertial mass in cantilever fashion; e. means fastening said second element to said inertial mass in cantilever fashion, said first and second elements having different points of attachment to said inertial mass, and extending outward in the same direction with respect to said inertial mass; and f. electrodes coupled to said sound-electrical transducer material of each of said first and second elements.
 15. A hydrophone responsive to sound energy for producing a voltage, said hydrophone adapted to be surrounded by a sound transmission medium comprising: a. an inertial mass; b. a relatively thin plate having main surfaces bounded by edges, having sound-electrical transducer material on at least one of said main surfaces, and having electrodes connected thereto; c. means fastening said plate to said inertial mass, said plate extending from said inertial mass so that said plate can be vibrated relative to said inertial mass by acoustic energy impinging on said hydrophone thereby causing a voltage to be produced at said electrodes in response to any of a plurality of spaced energy sources in said medium; and d. means for encapsulating said hydrophone.
 16. A device responsive to sound energy for producing a voltage in response thereto and responsive to a voltage for producing sound energy in response thereto comprising: a. an inertial mass; b. a relatively thin transducer element of piezoelectric material having main surfaces bounded by edges and having electrodes connected to main surfaces, said transducer element producing a voltage at said electrodes in response to forces applied to said main surfaces thereof and producing forces in response to a voltage applied to said electrodes; c. means fastening said transducer element to said inertial mass in the vicinity of one of said edges so that said transducer element extends free of other connections from said inertial mass in cantilever fashion; and d. means encapsulating said transducer element.
 17. A device responsive to sound energy for producing a voltage in response thereto and responsive to a voltage for producing sound energy in response thereto comprising: an inertial mass; a relatively thin transducer element of piezoelectric material having main surfaces bounded by edges and having electrodes connected to said main surfaces, said transducer element producing a voltage at said electrodes in response to forces applied to said main surfaces thereof and producing forces in response to a voltage applied to said electrodes; means for limiting the electrically effective area of each of said electrode to a value substantiallY less than the corresponding area of each said main surface; and means fastening said transducer element to said inertial mass in the vicinity of one of said edges so that said transducer element extends free of other connections from said inertial mass in cantilever fashion; said limiting means comprising nonconductive surface portions abutting said electrodes whereby the voltage to interelectrode capacity ratio characteristic is substantially increased as compared to the same transducer element having coextensive main surfaces and electrodes.
 18. The device of claim 17 wherein said limiting means comprises nonconductive surface portions abutting said electrodes and adapted to improve the voltage to interelectrode capacity ratio characteristic as compared to the same transducer element having coextensive main surfaces and electrodes. 